Sound transmission



DEC. 8, 1931. p MASON 1,835,795

v SOUND TRANSMISSION Filed June 14, 1929 lllllllllll-lll IIIIIIIIIII IIIATTORNEY Patented Dec. 8, 1931 RVARREN 'P. MASON, OF EAST ORANGE, NEWJERSEY, ASSIGNOR TO BELL TELEPHONE LABORAEORIES, INCORPORATED, OF NEWYORK, N. Y., A CORPORATION" OF NEW OBLK SOUND TRANSMISSION Applicationfiled June 14, 1929. Serial No. 370,815.

This invention relates to acoustic transmission systems and particularlyto .means fol-incorporating a sound wave channel into a telephonesystem.

7 .An object of the invention is to improve the speech transmissioncharacteristics of composite electrical-acoustic systems.

Another object is the elimination of wave reflection effects at thejunctions of acoustical and electrical transmission lines.

Aiurther object is to improve the acoustic damping in compositeacoustical-electrical systems.

.To produce a time delay in the :transmission of speech through atelephone system, or to modify the transmission in other ways, acousticdevices such as delay channels or acoustic wave 'filters have been used,these being inserted into the line as acoustic links and coupled theretoby telephone receivers or other suitable translating devices. Ifthe'translatlng devices were able totransfer the wave energy from theelectrical to the acoustical'portion of thesystem,=or vice versa, withperfect efliciency it would be poss1bleto proportion theinsertedacousticdevice so that,

Without the use oi additional elements,the .impedances oftheelectricaland the acoustic lines would be matched at t-he junction points, forminga homogeneous system through which wave energy can flowwvithout reflec-131011 and without loss. .been;poss1ble .to produce translating devicesHowever it has not having,eiiicienciesgreater than fifty per cent andeificiencies o't much lower values arethe general rule except inspecially constructed telephone receivers. Asa result of this theimpedances of the electrical and the acoustical lines cannot be directlymatched and disfltortion of the speech transmission characteristic duetoreflection occurs.

vibrating system "for which suitable damping must be provided to smoothout the resonance peaks .in the transmission characteristic. IVithperfect eiiiciency ofthe energytransfer this damping can be providedby-the electrical lines, but with low eificiency. energy'transthispurpose. In acoustic systems like loud speaking telephones, where ahorn-is-provided to radiate thesound waves into the air,

the radiation resistanceot the=horn mouth canbe made to furnish therightdegree of damping, but in acoustic links inatelephone line nodamping-of this kind isavailable. In accordance with thisinventionacoustic damping means is provided at oneor both ends of theacoustic link having ail-acoustic impedance complementary to that of thetranslating device. and the connected line in gether with the air,chamberin fIOIll/OfllllQ .f

diaphragm are proportioned to constitute a fer'the electrical damping isinsuificient for [iii Jill

mechanical transmission line having a char-- acteristicimpedance equalto thatof an air column-of the, same diameter-as the'outlet aperture infront'of the diaphragm.- The construction of matched impedance systemsofthis kind is described 'in the Bell System Technical Journal, 'vol. V,No. 3, -July 1926,

Methods of high quality recording and reproducing of music andspeechbased on'telephone research, by J. PjMaXfield-and I-I. C.Harrison. It has been foun-d'thatthe acoustical impedance of *a.telephone receiver of this type corresponds "very closely tothe i111-ped-ance of a short length of the uniform acoustic line defined by thereceiver opening, terminated by a resistance of value considerablydifferent from the characteristic impedance of the line. This propertyof the impedance leads to a simple type of compensating arrangementcomprising a similar short length of conduit branching from the mainacoustic line, in combination with actustic resistance elementsproportioned and arranged in the manner hereinafter described.

A feature of the invention is that the impedance compensating meanstogether with the translating device constitute an acoustic constantresistance structure.

Of the attached drawings,

Fig. 1 illustrates an acoustic transmission system in accordance withthe invention;

Fig. 2 shows in detail the structure of the terminal devices of thesystem of Fig. 1;

- Figs. 3 and 3A illustrate the construction of an element of theterminal device;

Fig. 4 is a schematic impedance diagram of the terminal device; and

N Fig. 5 is a typical impedance characteristic of a sound translatingdevice adapted to be used in the invention.

The'system of Fig. 1 is a composite electrical acoustical linecomprising a sound wave conduit having similar telephone receivers 11andll' coupled to its ends by means of which the conduit is included asan acoustic link between electrical lines 13 and 1d. The receivers arepreferably of the matched impedance type in which the vibratory elementsfrom a mechanical line of characteristic impedance equal to that of theconnected sound conduit described in the above mentioned article byMaxiield and Harrison. The windings of the receivers are connected tothe electrical lines through impedance matching transformers 12 and 12.At the junctions of the receivers with the sound conduit impedancecompensators 15 and 15 are provided in accordance with the invention tocoinpensate for the impedance disparitydue to the imperfect efiiciencyof the energy transfer in the receiver. In the figure the acoustic linkis shown as a simple unifrom tubular conduit such as would be used forproducing a time delay in transmission, but other types of acoustic linkmay be used within the scope of the invention, such, for example as theacoustic wave filters disclosed in U. S. Patent 1,692,317, issuedNovember 20, 1928 to G. W. Stewart. The construction of the impedancecompensating device is shown in de tail in the sectional. view of Fig.2. It comprises a housing in the form of a T-shaped pipe section havinga cross branch 16 which is threaded at each end for coupling to the faceplate of the receiver and to the sound conduit, and a side branch 17forming a wave path to the open air. In the cross branch, between theside branch and the receiver, is inserted an acoustic resistance element18 and in the side branch additional resistance elements 19 and 20 areinserted, one at each end. These resistance lements are preferably ofthe type in which the resistance is provided by a multiplicity of airpassages in the form of very narrow parallel slots, the width of theslots being such that the air motion therein is controlled soiely by theviscosity of the air. One suitable form is illustrated in Fig. 3,comprising a spider frame 21 having a circular rim for mountingpurposes, and a closely coiled spiral 22 of fine metallic ribbon mountedon one side of the frame. The resistance value is determined by thetotal slot area, the width of the slot, and the width of the ribbon. Toobtain a desired resistance value the element may be constructed bywinding a coated metallic ribbon of suitable dimensions and thickness ofcoating into a tight spiral and then dissolving off the coating.

With the impedance compensator inserted two sound wave paths areprovided at each terminal point of the acoustic line, the one path beingthe direct path through resistance 18 to the receiver and the other pathincluding resistance 19, the branch conduit 17 and the resistance 20.The manner in which these elements are proportioned is explained below.In Fig. t the impedance relations of the terminal apparatus areillustrated by the schematic of the analogous electrical circuit. Thesystem comprises two parallel circuits 21 and 22 correspondingrespectively to the acoustic paths to the receiver and in the sidebranch. Circuit 21 consists of a resistance R in series with animpedance Z, R corresponding to the acoustic resistance 18 and Z to theacoustic impedanceof the telephone receiver 11. Circuit 22 consists of aresistance R in series with an impedance Z R corresponding to theresistance 19 and Z to the impedance of the branch tube 17 terminated byresistance 20.

The impedance Z of the system is expressed by:

Z gR1+ Z1) (R2 Z2) R R Z Z Z;, S P Ip mechanical ohms,

or c. g. s. units,

where P represents atmospheric pressure; Y represents the ratio ofspecific heats of air;

p represents the density of air; S represents the cross-sectional areaof the pipe.

The value of 'l/ o p The. resistance elements 18 and 19 should thereforehave resistances equal to the characteristic impedance of the sound conduit and the branch conduit 17 in combination with the terminalresistance 20 should have an impedance inversely related to the receiverimpedance by the constant fact0! ZLZ.

In a telephone receiver of the matched impedance type the vibratingelements constitute the equivalent of a uniform mechanical transmissionline having a characteristic impedance which matchesthat of the uniformsound conduit of the same cross-sectional area as the opening from theair chamber in front of the diaphragm. The manner of proportioning thevibrating parts to obtain this result is described in .the hereinbeforementioned article by Maxfield and Harrison. It follows then that themechanical portion of the receiver is substantially equivalent inimpedance to some finite length of the sound conduit to which it isconnected. The damping effect of the connected electrical circuits andcorresponds to a resistance termination, but,

due to the imperfect efiiciency of energy transfer, the value of theresistance is too small to provide the proper termination for impedancematching. The value of the re ceiver acoustic impedance Z may be closelyrepresented by the formula sinh 1 5+; cosh Pl Z1 ZL-I cosh Pl+ sinh Plwhich is the expression for the impedance of a finite length of thesound conduit to which the receiver is coupled, terminated by a re- Theinversely "related impedance :required in theside branch of thecompensator isfound from equations (4) and (5) to be sinh Pl-l-r cosh.Pl (6 cosh Pl r s inh Pl This evidently represents the impedance of thesame length of conduit,but with-aresist ance termination 4 times asgreat asthe characteristic impedance. The compensation can therefore beaccomplished by the structure shown in Fig. 1.

To determine the proper length of the branch conduit 17 and the propervalue of .the resistance 20 it'is best to proceed by determining theacoustic impedance characteristic of the receiver andthe connectedelectrical circuit. This may be done by the method described in U. S.Patent.1,795,647,

issued March 10, 1931, to 1 BLFla-nders.

The impedance characteristic of -9. typical high efiiciencyreceiver isshown'in .F'ig. 5 in which the solid curve 23 represents the resistancecomponent and the dotted curvefZsl the reactance component. Thisreceiver'had an air chamber opening of 0.7 inch diameter and wasdesigned to operate with a sound conduit ofthis diameter or with ahornof the same throat diameter. The presence of the reactance andthevariation of the resistance are due to the improper iterminationprovided by the electrical circuits. The proportions of the equivalentsound conduit and terminal resistance are readily determined from theimpedance curves. .From equation resistance of the receiver impedanceandthe former tothe minimum. The length of'the equivalent conduit andhence also the length of the compensator conduit 17 is determined fromthefirst resonance frequency of the receiver, that is, the firstfrequency at'which the *reactancepasses through zero. At this frequencythe sound wave is "four times as'long as the equivalent conduit.

'For "the particular case illustrated by the curves of Fig. 5, themaximum resistance,

taking the average ofthe two peaks is 230 c. g. s. and'the'minimumis'50.From these values the characteristic impedance of the equivalent conduitis found to be 107 c. g. s.,

and the ratio r to be 2.14. The characteristic impedance'issthat ofacon'duit of 2.6 square 'cms. cross-sectional area, or approximately"0.7 inch diameter. The lengthof the equivalentconduit as determinedbyrfirst resonance .frequency,.750 c.; p. -s. is-equal to 4.5;inches.

including a second acoustic resistance A compensatorof the type of Fig.l for this receiver should therefore have a side branch 4.5 inches longand 0.7 inc diameter. Resistances 18 and 19 should each have the values107 c. g. s., and resistance 20 should have the value 230 c. s;

For the design of acoustic resistances of the coiled ribbon typedescribed above the following formula for the resistance value isconvenient R is the resistance value, ,a is the coefiicient of viscosityof air, t is the transverse width of the slot, (Z is the width of theribbon, A is the total slot area, and A, is the area of the conduit inwhich the resistance is user it is convenient to design the resistancefor a somewhat lower value than is desired and to adjust the element toits final value by blocking off part oi the slots to reduce the area A.

"What is claimed is: 1 i. in a wave transmission system an eiectricalline, a sound wave conduit, translating means coupling saidjconduit andsaid line andacoustic iinpecmnce means between said conduit and saidslitting device for compensating the dispa ity between thecharacteristic impedance oi the sound conduit and the acoustic impedanceof the translating device one to imperfect efii fiency of energytransfer in said translating device. 2. In combination, an electricalline, a sound wave conduit, energy translating means coupling saidconduit and'said line, the impedance of the combination of the A 0. g.s.

where translating means and the connected line pedance diiferent fromthe characteristic impedance or". said line, and an acoustic impedancecompensator between said line and said terminal device comprising anacoustic resistance directly in series with the terminal device and aside branch from the acoustic line i in series with an acousticimpedance inversely proportional to the impedance of said terminaldevice.

1. A combination in accordance with claim 3 in which the said acousticresistances each have values equai to the characteristic impedance 01the acoustic line and in which the acoustic impedance included in saidside branch is inversely related to the impedance of the said terminaldevice by the square of the characteristic impedance of the acousticline. r v

5. In combination, an acoustic transmission line, a sound translatingdevice connected to one end of said line, said device having an acousticimpedance corresponding to that of a finite length of said lineterminated by a resistance different from the characteristic impedanceof the line, and an acoustic impedance compensator between said line andid device, comprising an acoustic resistance in series with said deviceand an acoustic side branch consisting of a length of sound conduitequal to that defined by the impedance of the translating device, theinner and outer ends or" said side branch being closed through acousticresistances.

In witness whereof, I hereunto subscribe my name this 13th day of June,1929.

l VAR-REN P. MASON.

